Automotive seat belts in conventional usage have employed a spring-loaded retractor for holding the belt against the body of the occupant and normally allowing the belt to be released to permit movement of the occupant. In the case of sudden vehicle deceleration caused by braking or by a frontal impact, for example, an inertial latch secures the retractor to prevent belt feeding to thereby restrain the occupant. When a vehicle is equipped with supplemental inflatable restraints (SIR) a pretensioner is sometimes added to the seat belt. The pretensioner, when activated, provides a force to rapidly retract the seat belt to hold the occupant firmly against the seat back. The activation is provided in response to an electrical command signal when a frontal impact is deemed by the SIR micro-controller to be in progress. The motor for the pretensioner may be, for example, a pyrotechnic charge which is fired by the command signal to reel in the belt. Since the vehicle must be serviced to replace such a device after discharge, it is important that it be fired only when it is highly probable that a severe impact is in progress. Accordingly, the pretensioner is activated at the same time the SIR is deployed or slightly prior to deployment in response to a predetermined level of frontal impact.
It is desirable to extend the application of seat belt pretensioners to occupant protection during a rear impact. When rear impact is sufficiently severe, the occupant initially presses into the seat back and then springs forward, or sometimes forward and upward, to potentially make contact with the steering wheel or dashboard or with the ceiling. It is important that any control for activating the pretensioner upon rear impact does not interfere with its intended operation during a frontal impact, and that other types of impact do not trigger the activation. The other types of impact may be, for example, vibrations due to rough roads, contact of the undercarriage with the road surface or some object on the road, or hammer blows delivered to the vehicle during servicing.